Punch or stamping presses are a staple of manufacturing operations for products formed from workpieces such as metal sheets, rods, bars, etc. With properly designed tooling sets, punch presses can be used for a variety of manufacturing process operations including cutting, forming, drawing, shaping, and assembling. Punch presses come in sizes ranging from a meter or less in height to several meters in height, and can develop force ranging from hundreds to many thousands of kilograms.
The structure of a punch press includes a frame with a table and with a drive rod or shaft mounted on the frame. Force applied to the rod causes the rod to translate toward and away from the table. The table supports a fixed half of the tooling set called a die. The drive rod carries at an end adjacent to the die, a movable half of the tooling set called a punch and designed to closely mate or engage with the die. Punch presses are designed so that tooling can be easily replaced. An actuator mounted on the frame applies a large amount of force to the drive rod during a power stroke to move the drive rod and the punch carried by it toward the table. During each power stroke the actuator drives the punch toward the die to mate with the fixed die, with a workpiece between the punch and die. As the actuator forces the punch and die together, cooperating patterns in the punch and die bend, cut, draw, thin, etc. the workpiece as desired to create the intended product. Some tooling sets are designed with a number of stations so that the workpiece may be shifted sequentially to each of the stations between pressing events to complete the product.
The tooling set is made from tool steel or other hard, durable material. The tooling set must have precision dimensions and its halves are designed to mate with great accuracy as well as to operate without failure for many cycles under the high forces generated by the press. In fact, tool making is itself a recognized craft, with those having such skill in great demand. A tooling set must be designed to be compatible with the press for which it is intended. Design considerations include the amount of force each pressing operation requires and the amount of force the press can develop. By designing the tooling set for compatibility with the press and workpiece, a wide variety of products can be produced efficiently and economically.
The actuator traditional press designs use includes a heavy flywheel mounted for rotation on the frame in combination with a mechanical linkage and a clutch to develop and convert flywheel momentum to force applied to the drive rod. An electrical motor spins the flywheel up to a design speed. After the flywheel is spinning at its design speed, the operator engages the clutch, transferring the flywheel momentum to the mechanical linkage. The mechanical linkage applies the flywheel momentum to the drive rod to force the die halves to mate. On continuing rotation of the flywheel the linkage engages the drive rod to lift the punch from the die, allowing the operator to remove the finished workpiece. It is also possible to provide for a spring which is compressed during the power stroke, to provide for returning the drive rod once the clutch disengages.
The following is well known, but is helpful to clearly define a number of terms which will be frequently used hereafter, and to explain the basics of hydraulic cylinder operation. We use the term “hydraulic cylinder” or more conveniently, “cylinder” here to mean a hydraulic device for converting a flow of pressurized hydraulic fluid to linear mechanical motion, or for converting linear mechanical motion to a flow of pressurized hydraulic fluid. A cylinder comprises a housing having internal walls defining a cylindrical bore essentially closed at one end and open at the other, and with a port in the closed end through which pressurized hydraulic fluid flows. A piston which fits closely to and slides within the bore, defines a cylindrical pressure chamber between itself and the closed end of the bore. The pressure chamber is completely filled with hydraulic fluid. The volume of both the pressure chamber and the hydraulic fluid within the chamber changes as the piston slides within the bore. A piston rod is attached to the piston to transfer force between the piston and an external machine. When a cylinder operates in power mode, pressurized hydraulic fluid is forced into the pressure chamber through the port during a power stroke. During a power stroke, the piston slides linearly from a retracted to an extended position as pressurized hydraulic fluid flows into the chamber. A hydraulic pump of some kind provides the pressurized hydraulic fluid to the chamber.
Newer punch press designs use a hydraulic cylinder as the actuator, and the invention here forms an improvement to these hydraulic presses. The pump which supplies hydraulic fluid to the cylinder is attached to the frame along with the valves and other components of the hydraulic actuator system. Often the pump comprises a hydraulic cylinder operating in pump as opposed to force mode. The pump can draw its energy to operate from a compressed air source.
Of course, some mechanism must be provided for a hydraulic press to restore the piston to its retracted position after a power stroke. For hydraulic actuators, pneumatic or hydraulic pressure applied to the piston on the side opposite the pressure chamber can be used to force the piston to its retracted position. A spring can also be used to provide the retraction force.
In some designs the hydraulic “pump” for a hydraulic press's actuator comprises a so-called air over oil cylinder. An air over oil (AOO) cylinder has a piston which has a compressed air pressure chamber on one end and a hydraulic pressure chamber on the other end. High pressure air entering the air chamber drives the piston to force hydraulic fluid out of the hydraulic chamber and into the actuator hydraulic cylinder. By changing diameters of the pistons appropriately, the force provided by the compressed air can be greatly increased at the output of the hydraulic cylinder. An AOO cylinder-type hydraulic pump provides a moderate amount of high pressure hydraulic fluid inexpensively and with easily controlled pressure.
We find that traditional mechanical actuators have a number of problems in their operation. Among the problems are double strikes, faulty tool alignment, and operator risk. Double strikes for a mechanical press arise when a clutch improperly or unexpectedly applies force to the drive rod to mate the punch with the die without deliberately engaging the clutch. Typically, double strikes occur as the result of wearing or faulty adjustment of the clutch parts. Punch press clutches transfer large amounts of force and operate in dirty and otherwise hostile environments, so it is not surprising that the clutch mechanisms deteriorate with time. In the best of situations, proper maintenance prevents this deterioration, but in the real world proper maintenance does not always occur. And of course unseen and catastrophic failure of critical parts can also lead to double strikes.
Double strikes have the potential to be dangerous. If the operator's hand is between the punch and die for the purpose of removing the finished workpiece from the die, a double strike may smash the hand with obvious potential for serious injury. A less harmful scenario finds the operator's hand safely out of the danger zone but with the workpiece only partially removed or inserted. A double strike in this situation of course spoils the finished workpiece or workpiece blank, and may even damage the tooling.
Faulty tool alignment is a situation where the punch and die do not properly align. This usually also arises from wear or poor maintenance. The result is potentially to damage or even destroy the punch or die, or perhaps to damage the workpiece. Tooling under high loads has even been known to shatter causing broken parts to strike the operator. Even if there is no injury, the damage or destruction of a tooling set is quite enough harm to justify avoidance.
Operator risk occurs of course in the double strike situation as already mentioned. But even during normal operation, it is possible for an operator to carelessly leave her hand in the danger zone. Further, mechanical presses are extremely noisy, which has the potential for hearing damage to the operator. Ear protection reduces this possibility of harm, but makes it more difficult to speak to the operator, which has its own safety problems of course.
Hydraulic actuators have a number of advantages over mechanical actuators. First of all, there are fewer double strikes because the hydraulic and compressed air subsystems tend to deteriorate more slowly and less catastrophically. For example, a compressed air valve may fail by slowly leaking, which conceivably will give an operator time to shut down the press or at least remove her hand from the danger zone. However, the basic hydraulic actuator design does not absolutely preclude double strikes. For example, a malfunctioning compressed air valve can cause a double strike. Nor does a hydraulic actuator deal either with an operator's hand in the danger zone during normal operation, or with tool misalignment.
As to noise, the hydraulic press appears to be much more acceptable than the mechanical press. A hydraulic actuator is much quieter because the high force impact of a clutch arm striking a force-transferring surface on the drive rod is eliminated.
So the present state of the art is that hydraulic cylinder type actuators provide large forces inexpensively and somewhat more safely than mechanical actuators. For this reason they are becoming quite popular for presses. However, they (and mechanical actuators as well) still have significant disadvantages. The enormous forces which these presses apply to the workpiece have the potential to cause serious operator injury. A number of safety features have been devised to prevent operator injury. While these are usually effective, they tend to slow down production, are not always effective, or can even be defeated by careless or rushed operators. Accordingly, it is fair to say that presently available designs do not completely resolve punch press safety issues.